The proposed research represents a continuation of the investigation on the transport of calcium and other mineral ions across the intestine. Emphasis will be given to vitamin D and other factors that control mineral ion absorption and intestinal function. Specific studies to be undertaken within this grant period are, as follows: a) Continued investigation of the function vitamin D and calbindin-D28K in Ca2+ transport across epithelial membranes and continued investigation of the ATP-dependent calcium pump, with emphasis on the responsiveness of the pump to vitamin D and calbindin-D28K; b) Investigate the state and localization of calcium and other mineral ions during the course of transepithelial transport using spectrofluorometry with fluorescent calcium indicator dyes (fura 2; quin 2) for free Ca2+, and ion microscopy for total Ca2+. Chlortetracycle, another fluorescent Ca2+ indicator, will be examined as a probe for intravesicular Ca2+; c) Investigate the mechanism or mechanisms of magnesium transport across the intestine and other epithelial tissues, and the role of vitamin D and other affectors on its transport and metabolism; d) Investigate the effect of vitamin D on the function of the intestinalis nerve of the chick, the latter shown to contain vitamin D-responsive calbindin-D28K, using various neurophysiological and biochemical approaches. e) Continued studies on the effect of E. coli heat stable enterotoxin and cGMP on the Ca2+ permeability of the enterocyte, and f) Continued studies on the mechanism of selenite absorption and the relation of vitamin D-dependent selenite absorption and the relation of vitamin D-dependent selenite transport to other anion transport systems.